The present studies are a continuation of those designed to study the effect of modifying cell surfaces or their immunologic behavior. We have shown that Vibrio cholerae neuraminidase (VCN) and Concanavalin A (ConA) increases the immunogenicity of malignant cells. Because of this property, total regression of firmly established tumors can be induced in syngenic mice and in spontaneous mice by inoculation of living tumor cells treated with VCN or ConA. Experiments are currently designed to study tumor regression in other models utilizing different tumors in different sites and utilizing tumors treated with chemotherapy and immunotherapy combined. The cellular constituents of the regression tumor will be analyzed by looking for activated macrophages within the tumor and, utilizing models of in vitro cytotoxicity recently worked out, the in vitro capacity of lymphocytes from animals bearing regressing tumors to kill tumor cells will be studied. Newer studies are planned to investigate further the augmented in vitro generation of cytotoxic cells in the presence of neuraminidase. BIBLIOGRAPHIC REFERENCES:Simmons, R.L., Toledo-Pereyra,L.H., Nowygrod, R., and Najarian, J.S.: Masking of kidney-graft antigens with concanavalin A. Transplant. Proc. VII (Suppl.l):677, 1975. Toledo-Pereyra, L.H., Callender, C.O., Ray, P.K., Najarian, J.S.,and Simmons, R.L.: Effect of concanavalin A and phytohemagglutinin on the modification of immunogenicity of canine kidney allografts. Adv. Exp. Med. & Biol. Vol. 55:(Concanavalin A, edited by T.K. Chowdhury and A.K. Weiss): 309: 1975 (Plenum Press, New York).